U.S. patent number 4,722,021 [Application Number 06/915,919] was granted by the patent office on 1988-01-26 for safety circuit for hand tools, and method for safe operation thereof.
This patent grant is currently assigned to Robert Bosch GmbH. Invention is credited to Martin Gerschner, Klaus Gunther, Friedrich Hornung, Fritz Schadlich.
United States Patent |
4,722,021 |
Hornung , et al. |
January 26, 1988 |
Safety circuit for hand tools, and method for safe operation
thereof
Abstract
To detect if a tool bit, such as a drill, a saw blade, or the
like, upon being worked into a wall, meets a metallic obstruction,
such as a conduit, reinforcement rod, hydraulic pipe or the like,
an alternating voltage, preferably between 2 and 20 kHz, is coupled
through a coupling capacitor (9) to the tool bit, and current flow
to the tool bit is detected by a current measurement stage (10; 31,
32) to disconnect energy to the drive motor (5), for example by
disabling firing of a thyristor (15), or closing a valve of a
compressed-air tool. The coupling capacitor should have a capacity
small enough to provide high impedance to network power frequency,
and to insure a substantial change in current being supplied to the
tool bit holder, typically a chuck, if the tool bit meets a
metallic object in the wall. The a-c supply circuit can be formed
as part of a trigger circuit for the thyristor, by pulse-energizing
a trigger diode (28) upon repetitive charge of a capacitor (26)
through a charging network (22, 23, 24, 25).
Inventors: |
Hornung; Friedrich (Stuttgart,
DE), Schadlich; Fritz (Leinfelden, DE),
Gerschner; Martin (Leinfelden-Echterdingen, DE),
Gunther; Klaus (Stuttgart, DE) |
Assignee: |
Robert Bosch GmbH (Stuttgart,
DE)
|
Family
ID: |
6173938 |
Appl.
No.: |
06/915,919 |
Filed: |
October 3, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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524686 |
Aug 19, 1983 |
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Foreign Application Priority Data
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Sep 23, 1982 [DE] |
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3235194 |
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Current U.S.
Class: |
361/49; 361/100;
361/180; 361/181; 361/42; 388/831; 388/903; 388/909; 388/919;
388/937 |
Current CPC
Class: |
B23B
45/00 (20130101); B28D 1/14 (20130101); F16P
7/00 (20130101); H02H 3/14 (20130101); Y10S
388/919 (20130101); Y10S 388/903 (20130101); Y10S
388/937 (20130101); Y10S 388/909 (20130101) |
Current International
Class: |
B28D
1/14 (20060101); B23B 45/00 (20060101); F16P
7/00 (20060101); H02H 3/14 (20060101); H02H
003/17 () |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leung; Philip H.
Assistant Examiner: Williams; Howard L.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Parent Case Text
This application is a continuation of application Ser. No. 524,686,
now abandoned, filed Aug. 19, 1983.
Claims
We claim:
1. Method of deenergizing an electric power tool connected to a
two-wire power supply connection having
an electric motor supplied by network power at network frequency
from said two-wire power supply connection and having a metallic
bit holder (6) holding a metallic cutter bit therein, said
deenergizing method being effective if the bit inadvertently
contacts a metallic object (16, 17) regardless of whether said
metallic object is positively grounded,
comprising the steps of
energizing said power tool with power at power network
frequency;
furnishing a test circuit supply voltage;
utilizing said test circuit supply voltage for generating an
alternating electrical test energy of a frequency substantially
elevated above power line frequency, and having a range of between
2 and 20 kHz;
connecting the alternating electrical test energy at a voltage
level of substantially below power line voltage to said bit holder
by a capacitor (9) having a coupling impedance which is high with
respect to power line frequency and an impedance which is low with
respect to said elevated frequency of the electric test energy;
coupling the electrical test energy to at least one of the wires of
the two-wire power supply connection;
sensing the level of electrical test energy at said elevated
frequency applied to said bit holder;
measuring change in said sensed electrical test energy applied to
the bit holder, and then towards said metallic object; and
controlling energization of the power tool by disconnecting
energization thereof if change in test energy applied to the bit,
above a predetermined change level, is sensed.
2. The method of claim 1, wherein said step of generating said
electrical test energy comprises generating test energy at a
frequency of about 12 kHz.
3. The method of claim 1, wherein the metallic object is located in
non-metallic objects, said non-metallic objects comprising
substantially non-conductive building materials of the group
consisting of at least one of: cement, plaster, gypsum board,
gypsum blocks, mortar, bricks; and
wherein the capacitor has a capacity of about 100 pF.
4. Safety circuit used in power tools, particularly hand tools,
used in deenergizing the power tool upon contact of a bit with a
metallic object (16, 17), regardless of whether said metallic
object is grounded, said power tool having a two-wire power supply
connection and an electrically conductive bit holder (6),
comprising
a source of electrical test energy having a frequency elevated
substantially above power line frequency and a test voltage
substantially below power line voltage, said source having two test
energy output terminals;
a capacitor (9) applying said test energy from one terminal of the
electrical test energy source to the bit holder (6), said capacitor
(9) having a coupling impedance which is high with respect to power
line frequency and an impedance which is low with respect to the
elevated frequency of electric test energy;
coupling means (34; 22-25, 26) for high frequency coupling the
other terminal of the test energy source to at least one wire of
the two-wire power supply connection;
a test current measuring circuit (10, 30, 31) connected between the
electrical test energy source and the capacitor (9) and sensing
change of current to said bit and bit holder; and
a disconnect circuit connected to and controlled by the test
current measuring circuit (10) and disconnecting energization from
said two-wire power supply connection to the power tool when the
test current measuring circuit (10) detects change in test current
above a predetermined level, upon contact of the bit with said
metallic object (16, 17).
5. The circuit of claim 4, wherein said electrical energy source
comprises an alternating current or pulse voltage source.
6. The circuit of claim 4, wherein said electrical energy source
comprises a high-frequency generator.
7. The circuit if claim 4, wherein said energy source comprises a
high-frequency generator operating at a frequency in the range of
between about 2 to 20 kHz.
8. The circuit of claim 4, further comprising a memory element (13)
connected to and controlled by the current measuring circuit and
storing a signal delivered by the current measuring circuit
indicative of current flow above said predetermined level.
9. The circuit of claim 4, wherein the power tool has an electric
motor (5);
a semiconductor switch (15) is provided to control energization of
the motor;
a trigger circuit is connected to the semiconductor switch for
triggering said semiconductor switch;
and wherein the test current measuring circuit is connected to and
controls the trigger circuit to prevent energization of the motor
when current above the predetermined level is detected.
10. The circuit of claim 9, further including a thyristor (29)
forming a memory connected to deenergize the motor of the power
tool, the gate of the thyristor (29) being connected to and
controlled by the test current measuring circuit to fire the
thyristor when the predetermined level of current is exceeded.
11. The circuit of claim 4, further including a self-holding
circuit (13; 29, 30) connected to and controlled by the test
current measuring circuit (10), and storing a condition when the
current is above the predetermined level, and further maintaining
said stored condition until the power tool is deenergized to
prevent inadvertent reenergization after the power tool has been
disconnected, without positive disconnection and reconnection
thereof to the power supply connection.
12. The circuit of claim 4, further including calibration means
(34, 37) connected to said test current measuring circuit to change
the response sensitivity thereof and thus control the predetermined
level at which test current flow is detected.
13. Safety circuit used in power tools, particularly hand tools,
used in deenergizating the power tool upon contact of a bit with an
electrically conductive object (16, 17), said power tool having an
electrically conductive bit holder (6) and an electric motor (5),
comprising
a high-frequency electrical energy source providing electrical
energy having a frequency substantially above power line frequency
and a voltage substantially below power line voltage;
coupling means (9) applying the voltage from the electrical energy
source to the bit holder (6);
a current measuring circuit (10, 30, 31) connected between the
electrical energy source (11) and the coupling means (9);
a disconnect circuit (14, 15; 24, 25, 26, 28, 29) connected to and
controlled by the current measuring circuit (10, 30, 31) and
disconnecting energization to the motor (5) when the current
measuring circuit detects current above a predetermined level, said
disconnect circuit including
a semiconductor switch connected to the motor (5);
a trigger circuit connected to the semiconductor switch (15) and
thereby controlling the motor (5);
the current measuring circuit (10, 30, 31) being connected to and
in turn controlling the trigger circuit; and
a direct current source providing power to the trigger circuit;
said trigger circuit including a trigger diode (28) and a capacitor
(26) charged by direct current from the direct current source, the
trigger diode (28) periodically discharging the capacitor (26) to
form therewith an oscillatory circuit;
and connection means coupled to the oscillatory circuit and forming
said high frequency energy source.
14. Safety circuit used in power tools, particularly hand tools,
for de-energizing the power tool upon contact of a bit with a
metallic object (16, 17) regardless of whether said metallic object
is grounded, said power tool having an electrically conductive bit
holder (6) and a two-wire power supply connection, comprising
a source of electrical test energy having a frequency substantially
above power line frequency and a test voltage substantially below
power line voltage;
a capacitor (9) applying said test energy from the electrical test
energy source to the bit holder (6), said capacitor (9) being
capable of effectively blocking power line frequency current;
a test current measuring circuit connected between the electrical
test energy source and a capacitor (9), and sensing change of
current to said bit and bit holder, said test current measuring
circuit comprising
a resistor (32) serially connected between the source of electrical
test energy and the capacitor (9);
a transistor (31) having its base-emitter path connected in
parallel to the resistor (32);
and a response circuit (30) including the collector of the
transistor (31), the transistor changing conduction state when the
voltage across the resistor (32) due to current flow to said bit
and bit holders exceeds the base-emitter voltage of the transistor;
and
a disconnect circuit (14, 15, 24, 25, 26, 29) connected to and
controlled by the test current measuring circuit and disconnecting
energization from said two wire power supply connection to the
power tool, when the test current measuring circuit detects change
in test current above a predetermined level, upon contact of the
bit with said metallic object (16, 17).
15. A power tool, particularly motor-driven hand tool, having
an electric motor (5);
two-wire power supply lines (3, 4) supplying electrical power to
said motor at network frequency;
an electrically conductive bit holder (6) driven by the motor;
a motor controlled triac (15) connected for energization of the
electric motor from said power lines; and
a triac trigger for rendering said motor controlled triac
conductive,
in combination with a safety circuit for deenergizing the motor
upon contact of a bit in the bit holder with an electrically
conductive object (16), said safety circuit comprising
a source of electrical energy coupled to at least one of the power
line conductors, and having a frequency substantially elevated
above power line frequency and a voltage substantially below power
line voltage;
capacitor means (9) dimensioned to have a high impedance at power
line frequency and a low impedance at said elevated frequency, and
applying the electrical energy from the electrical energy source to
the bit holder (6);
a current measuring circuit connected between the electrical energy
source and the capacitor means (9); and
a disconnect circuit connected to and controlled by the current
measuring circuit and disconnecting energization to the motor when
the current measuring circuit detects current at said elevated
frequency above a predetermined level,
said disconnect circuit including an oscillator circuit which
serves both as said triac trigger and as the source of said
electrical energy having said elevated frequency.
Description
The present invention relates to hand tools, and more particularly
to electric drills, saws and the like, which have tool bits or
blades which may come in contact with concealed metal objects, such
as nails, reinforcing rods, buried conduits, cables, pipes, and the
like, and to a method to prevent danger to the operator.
BACKGROUND
Saws, drills, and particularly hammer drills, when used to drill or
cut into walls, may inadvertently contact buried or concealed
cables, conduits, pipes, or nails, or, also, reinforcing rods which
may be inserted in cement walls. If a drill bit or saw
blade--hereinafter for short "bit"--meets one of these obstructions
within a wall, it is always possible that the conduits or pipes
will be damaged, for example by being cut or having a hole drilled
thereinto. Even if the armoring or metal used for the concealed
conduits, pipes and the like is sufficiently sturdy to deflect the
bit, or hard enough so as not to be damaged thereby, damage to the
bit itself is hardly avoidable. When sawing with a cross-cut blade,
and particularly with a fine-tooth blade, across wood, and a nail
is buried in the wood, the saw blade will be severely damaged, and
may be entirely destroyed. It was, heretofore, necessary for the
operator to be careful and immediately disconnect the tool as soon
as contact with a buried object having a hardness characteristic
which was not intended to be cut was detected. Skill and judgment
is required on part of the operator to notice such contact.
THE INVENTION
It is an object to provide a safety circuit, and a method for safe
operation of power tools, and particularly portable power tools,
such that the tool is disconnected as soon as contact with a buried
object occurs, or is imminent.
Briefly, an electrical test voltage is connected or coupled, for
example by a capacitor, to the tool bit holding chuck or to the
arbor portion holding a saw blade. Current flow through the
circuit, which includes the tool bit holder, that is, the chuck,
arbor, or similar element, is measured, and power energization is
controlled as a function of electrical energy flow in this circuit,
for example by sensing when a predetermined current is
exceeded.
In accordance with a preferred feature of the invention, the test
voltage being coupled to the chuck, arbor, or the like, hereinafter
for short "tool bit holder", is an alternating voltage and,
preferably, of high frequency, having a frequency for example in
the range of between 2 and 20 kHz. Since, most likely, objects
buried in the wall will be grounded, either through the wall itself
or through a galvanic grounding connection, and buried nails and
the like will have a substantial capacitance, that is, substantial
with respect to the remainder of the circuit, current will change
upon contact with the metal object. Since the tool bit is sharp and
rotates at high speed, even slight contact will immediately effect
metal-to-metal contact, cleaning off any accumulated dirt, rust, or
the like, so that the change in current flow in the circuit which
includes the tool bit holder can be instantaneously detected.
The circuit can be so arranged, and made so sensitive, that
galvanic connection of the metal object which might meet the tool
bit is not necessary. The high-frequency discharge current rises
sufficiently if the metal object has a reasonably sufficient
inherent capacitance, that is, a certain minimum size. A larger
nail would have such capacity.
The safety method and circuit or system is useful not only for
electrically driven power tools, but can be applied, equally, with
hydraulic or, typically, compressed-air tools. It is only necessary
to replace the electrical turn-off circuit used in an electrical
tool with a turn-off valve, perferably a quick-acting valve, which
controls energization of the fluid motor. If a fluid motor, such as
a compressed-air motor, is used, a separate battery, or an external
power network supply can be used to furnish the necessary
energization power for the high-frequency generator. The fluid tool
will, however, additionally require a grounding connection. Such a
grounding connection is desirable, in any event, for operator
safety.
The system has the advantage that the tool is automatically
disconnected as soon as a metallic object is touched, particularly
when grounded, or having a very large inherent capacity, such as
cement reinforcement rods, or conduits, hydraulic pipes and the
like, when the metallic object is closely approached. The system
has the advantage that hidden reinforcement rods, water and steam
pipes and the like, which may be concealed in walls, will not be
destroyed, and tools being used to drill or saw in the walls will
not be damaged or rendered useless. The system has the additional
advantage that it can be easily constructed and will reliably turn
OFF the power tool, powered by electricity or by a power fluid, for
example compressed air, even if the metal object being touched is
not grounded.
Use of alternating current, particularly of a frequency which is
high with respect to power network frequency, permits indication
and turn-off of the tool upon contact with ungrounded, concealed
metal objects.
The arrangement or circuit preferably includes a voltage source,
most suitably of between 2-20 kHz, in which electrical power is
conducted to the tool bit holder over a current measuring element.
When the current measuring element senses current flow above a
predetermined level, the power supply to the tool is interrupted.
If the tool includes an electric motor, a thyristor circuit can be
disconnected; if the tool includes a compressed-air motor, a valve
can be closed as soon as the current measuring sensing circuit
responds. The turn-off switches or valves, respectively, already
present in the tool, thus can be used, and the only additional
elements required are the high-frequency generator, sensor,
response circuit, and coupling to the tool bit holder. The voltage
source, preferably an alternating current voltage source, is
suitably connected to the tool bit holder via a capacitor.
If the drive motor for the tool bit is an electric motor, then, in
accordance with a preferred feature of the invention, a
particularly simple circuit can be provided, in which the trigger
circuit for the control thyristor or triac of the motor is used at
the same time as an oscillator circuit to supply the voltage to the
tool bit, the current flow through which is being sensed. The
oscillator circuit then, at the same time, can form a voltage
source for the tool bit holder and, at the same time, a control
circuit for the thyristor or triac which controls the tool
operation and, for example, its speed. This arrangement permits
reducing the number of structural elements which are required.
In accordance with a preferred feature of the invention, a memory
or storage element is provided which switches-over upon first
metallic contact with the hidden object and inhibits further
operation of the tool. This memory, most simply, is formed as a
thyristor. Resetting the memory can be effected then only by
disconnection of the current supply to the motor, for example by
unplugging the tool or conscious release of a main switch. The
operator, thus, is warned that the tool met with an
obstruction.
DRAWINGS
FIG. 1 shows the basic block diagram of the system to disconnect an
electrically operated power tool; and
FIG. 2 is a schematic circuit diagram of a particularly simple and
inexpensive turn-off circuit when using an electric drive
motor.
The invention will be described in connection with an electric
drill, for example a hammer, or percussion-type drill, in which a
drill bit is held in a chuck 6 of conventional construction, the
chuck forming the tool bit holder.
An alternating current supply, schematically shown at 1, and having
one terminal grounded as shown at 2, is connected over a
socket-and-plug connection 4, 3 to the power tool. The network
supply 1, for example, will supply power 110 V, 220 V, or the like,
at power frequency of, for example, 60 to 50 Hz. The terminals 4, 3
may, for example, symbolize the plug terminal of the electric drill
shown schematically in FIG. 1.
The drill 1 has a motor 5 which has one terminal connected to plug
terminal 4, in accordance with well known and standard connection.
The other terminal of the motor 5 is connected through a triac 15
to the second or grounded plug terminal 3. The drive motor 5 is
connected over a gearing, shown only schematically, to a drive
spindle 8 which terminates in a chuck 6. The drive spindle 8 is
journalled in a bearing 7.
A capacitor 9 is connected to the bearing 7 which, in turn, is
connected to a current measuring stage 10. The current measuring
stage is supplied with current by a high-frequency generator 11
which, for example, operates or oscillates at a frequency of
between 2 and 20 kHz, that is, substantially elevated above power
line frequency. A current supply 12 for the generator 11 is
connected across the terminals 4, 3 and supplies the generator 11,
as well as a memory circuit 13 and the trigger circuit 14 of the
triac 15 with a suitable low-voltage d-c supply. The memory circuit
13 controls operation of the triac trigger circuit 14 which is
connected to the gate of the triac 15.
A metallic object 16, connected to a metal sink 17 which may, but
need not be grounded, is additionally shown schematically. The
element 16 may, for example, be formed by a concealed reinforcement
rod in a cement wall, conduit, armoring, hydraulic piping or the
like--in short any metal object which the system, and method, of
the present invention is intended to detect.
Operation: Basically, the system has an electronically controlled
switch which has self-holding functions, due to the presence of the
memory circuit 13.
Oscillator 11 generates a high-frequency voltage. The wave shape is
not critical and may be sinusoidal, pulse-shaped with essentially
square-wave pulses, or the like. The voltage is connected through
the current measuring stage 10 to the coupling capacitor 9 which
applies the voltage to the bearing 7 which, then, transfers it to
the tool bit holder 6, typically a chuck. The voltage of the
high-frequency generator 11 preferably has a level of about 10 V,
with a frequency of between about 2 to 20 kHz.
Upon contact between the tool bit in the bit holder 6 and the metal
element 16, current I will flow to the metal sink or capacitance
17. The sink 17 may be directly connected to ground 2, or, if an
electrical conductor was touched by the bit, it may even be one or
the other of the terminals of the voltage source 1. If the metal
element is in a wall but insulated from ground, current will flow
over the impedance specific to the wall structure to the sink 17.
In any event, a substantial change in current I will result which
is sensed in the current measuring stage 10. When a certain current
level is exceeded, for example as determined by a threshold circuit
in the current measuring stage 10, a switching output pulse is
delivered which sets the memory 13. Upon receiving a SET pulse from
the current measuring stage 10, the memory 13 disables transmission
of trigger pulses to the triac 15 through the triac trigger circuit
14, and the triac 15 will no longer fire, thus immediately stopping
the machine.
Motor 5 will be de-energized until the memory 13 is reset.
Preferably, this is carried out by disconnecting network voltage,
and re-connecting network voltage. Memory 13 is so connected that,
when the internal supply voltage supplied by the d-c supply 12
collapses, the memory 13 reverts to its initial state controlling
the triac trigger 14 to provide trigger pulses, and the tool, then,
will be ready for operation. It is, of course, possible to provide
a separate switch or reset button to permit resetting the memory
without physically disconnecting the motor 5, by unplugging the
tool, or turning OFF a main switch or the like.
The capacitor 9, typically, has a value of about 100 pF. The
capacitor 9 is preferably selected to have a value which is small
enough that power network current is effectively blocked thereby.
The value of the capacitor 9 must be such that the high-frequency
current generated by generator 11 is readily passed thereby, but
power frequency current is effectively blocked. Otherwise,
legislated and mandated safety and insulation provisions may be
affected.
The frequency of the oscillator 11 should be so selected that the
reactance of the coupling or isolating capacitor 9 is small with
respect to the impedance of the walls or structures in which the
tool is being used. If the reactance of the capacitor 9 is too
large, the change in current flow to be detected by the stage 10,
will be difficult to distinguish between normal operation and an
encounter with an object, since normal drilling operation will
already cause current flow through walls and the like which is
sufficiently large so that additional current upon meeting a
metallic obstruction may be masked. Reliable response of the
current measuring stage 10 thus cannot be insured.
The respective elements thus should be so matched to each other
that galvanic connection of the metal object 16 to ground 2 is not
absolutely necessary. The high-frequency leakage current will then
rise sufficiently, provided the element 16 with the sink 17 has
sufficient capacitance, that is, a certain minimum value.
If the motor is a compressed-air motor, the triac 15 will be
replaced by a valve, preferably a quick-acting valve, supplying
compressed air to the motor, and the d-c supply 12 will be replaced
by a battery, or low-voltage power supply. A ground connection to
ground 2 of the compressed-air motor must be provided. Such a
ground connection is desirable for operator safety in any event,
and usually provided by a grounding cable or grounding clip on the
tool, or a separate connection on a compressed-air hose, for
example through an armor thereof.
FIG. 2 illustrates a particularly simple arrangement, suitable for
use with triac-controlled electric power tools. Terminals 4, 3 are
supplied, as in the embodiment of FIG. 1, with power network
frequency. Terminal 4 is connected to the motor 5, and through the
triac 15 then to terminal 3. Terminal 4, further, is connected to a
diode 22 which is serially connected with a resistor 23. The other
terminal of resistor 23 is connected to a series circuit of two
resistors 24, 25, which have a common junction 35. A capacitor 26
is connected between the other terminal of resistor 25 and forms a
junction 36 therewith. The remote electrode of the capacitor 26 is
connected to terminal 3. The junction 36 is connected to a trigger
diode 28 which, in turn, is connected with a gate of the triac 15.
Junction 36 is, further, connected to the emitter of a transistor
31 and to an emitter resistor 32 which is connected to the base of
the transistor 31. The base of the transistor 31, and the junction
with the resistor 32, is connected to one electrode of the
capacitor 9 which, again, is connected with the tool bit holder 6,
shown as a chuck, for example by being coupled to a bearing for the
chuck spindle. A resistor 34 and switch 37 are connected across the
resistor 32. Since the circuit 34-37 is not strictly necessary, it
is shown in broken line. The collector of the transistor 31 is
connected with the gate of a thyristor 29 and over a resistor 30 to
junction 35 to which, also, the cathode of the thyristor 29 is
connected. The anode of thyristor 29 is connected to the terminal
3. Filter capacitor 27 is connected across circuit
24-35-25-36-26.
Operation: The trigger circuit, which includes the trigger diode
28, resistor 25 and capacitor 26, is not supplied with a-c as is
customary, but rather via diode 22, and resistors 23, 24 with
direct or rectified current. Thus, the trigger will not supply
single pulses but, rather, a periodic pulse sequence of high
frequency. The frequency is determined by the capacitor 26 and the
resistor 25. Capacitor 26 is charged with direct current, and
discharges over trigger diode 28 and triac 15 always when the
voltage at the capacitor 26 reaches the trigger voltage of the
trigger diode. The trigger diode, thus, will have a high-frequency
current flowing therethrough and to the triac 15. The frequency,
between 2 and 20 kHz, e.g. 12 kHz, is substantially elevated above
power line frequency of, for example, 50 Hz. Triac 15 will be
ignited thereby. Additionally, the junction 36 will supply a
high-frequency voltage. Junction 36 is high-frequency coupled to
both terminals 3 and 4 (see FIG. 2). It is this high-frequency
voltage which is coupled via the current measuring resistor 32 to
the coupling capacitor 9 and then to the tool bit holder 6.
The trigger circuit for the trigger diode 28 thus has a dual
function: (1) to provide trigger pulses, and (2) to form the
high-frequency oscillator to supply high-frequency current to the
tool bit holder 6.
If the voltage drop on resistor 32 reaches the level of the
base-emitter threshold of transistor 31, for example when contact
with a metallic element 16 (not shown in FIG. 2) occurs by a tool
in the tool bit holder 6, current will flow through the
emitter-collector path of the transistor and thus cause voltage
drop across resistor 30, firing the auxiliary thyristor 29 which
short-circuits the voltage between junction 35 and terminal 3. This
de-energizes the self-oscillating trigger, and the thyristor 15 is
immediately disconnected.
The auxiliary thyristor 29 will be disconnected only when the
entire circuit is de-energized, that is, upon de-energizing
terminals 4, 3.
The sensitivity of the circuit can be controlled by calibration
means, for example, by selectively connecting resistor 34 by
closing switch 37; additional resistors and switches similar to
resistor 34 and switch 37 may be used.
The circuit arrangement is suitable not only for drills, for
example hammer or percussion drills or the like, but for any type
of power tool, particularly for portable tools. The circuit thus
can be used with circular saws, saber saws. The high-frequency
generator 11 may be replaced by a direct current source, in which
case the direct current voltage is applied to the tool holder bit
through a resistor, rather than through a capacitor 9.
Various changes and modifications may be made, and features
described in connection with any one of the embodiments may be used
with any of the others, within the scope of the inventive concept.
The term "bit" and "bit holder" is deemed to encompass not only
drill bits and the like, but any kind of cutter elements with which
the tool is intended to be used, for example a circular saw blade
and saw arbor, a saber saw blade, and blade clamp, an electrically
conductive, abrasive disk, and disk clamp, or the like.
In one operative embodiment, for a power network frequency of 50
Hz, with a supply voltage of 220 V, a suitable frequency of the
generator 11 is about 12 kHz For 220 V, 50 Hz power supply,
suitable circuit components for the embodiment of FIG. 2 are:
resistor 23: 15 K.OMEGA.
resistor 24: 10 K.OMEGA.
resistor 25: 2,2K.OMEGA.
capacitor 26: 0,047.mu.F
capacitor 27: 4,7.mu.F
resistor 30: 1 K.OMEGA.
resistor 32: 270.OMEGA.
coupling capacitor 9: 330 pF
* * * * *